MECHANISM OF HYDROGEN ABSTRACTION REACTIONS BY FREE-RADICALS - SIMPLE METATHESIS OR INVOLVING INTERMEDIATE COMPLEX
MECHANISM OF HYDROGEN ABSTRACTION REACTIONS BY FREE-RADICALS - SIMPLE METATHESIS OR INVOLVING INTERMEDIATE COMPLEX
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DOI：
10.1021/j100117a019
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发表时间：
1993-04-15
影响因子：
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通讯作者：
TSCHUIKOWROUX, E
中科院分区：
文献类型：
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作者：
CHEN, YH;TSCHUIKOWROUX, E
用从头算方法研究了R + HX --> RH + X（R = CFyH_3-y（y = 0-3），C_2 H_5; X = F，Cl，Br，CH_3）的夺氢反应.研究发现，当反应物在HX的氢上带有净正电荷，在自由基位点的碳上带有净负电荷时，反应通过弱氢键中间络合物（R.）进行。在过渡态（R.H.X）形成之前，在（U）MP2/6- 31 G * 水平上优化了反应物、中间体、过渡态的几何构型。在（U）MP4/6- 311 G **//（U）-MP2/6- 31 G * 水平上计算了电子相关能。对于反应CH_2F + HBr --> CH_3F + Br，采用Gaussian-1（G_1）理论。中间体络合物（R. H.X）主要取决于极化反应物之间的偶极相互作用以及自由基的SOMO与HX的σ *HX轨道之间的单电子双轨道相互作用。理论结果也间接支持了实验上发现的R + HBr --> RH + Br（R =烷基自由基）的负活化能。本文用过渡态理论（TST）和RRKM理论计算了C_2H_5 + HBr（DBr）→ C_2H_6（C_2H_5D）+ Br反应的绝对速率常数和动力学同位素效应。
Hydrogen abstraction reactions, R + HX --> RH + X (R = CFyH3-y (y = 0-3), C2H5; X = F, Cl, Br, CH3), have been investigated by ab initio methods. It is found that with reactants which are polarized with a net positive charge on the hydrogen of HX and a net negative charge on the carbon at the radical site, reactions proceed via a weakly hydrogen-bonded intermediate complex (R..H.X) prior to the formation of the transition state (R.H.X). All geometries (reactants, intermediate complexes, transition-state structures) were optimized at the (U)MP2/6-31G* level. Electron correlation energies were evaluated at the (U)MP4/6-311G**//(U)-MP2/6-31G* level. For the reaction CH2F + HBr --> CH3F + Br, the Gaussian-1 (G1) theory was employed. The stability of the intermediate complexes (R..H.X) depends mainly on the dipole interaction between the polarized reactants as well as the one-electron, two-orbital interaction between the SOMO of the radical and the sigma*HX orbital of HX. The theoretical results also furnish indirect support for negative activation energies found experimentally for R + HBr --> RH + Br (R = alkyl radical). Absolute rate constants and kinetic isotope effects for the reaction C2H5 + HBr(DBr) --> C2H6(C2H5D) + Br are evaluated by transition-state theory (TST) and RRKM theory as applied to the dissociation of the intermediate complex.